Kraft pulping is performed by cooking wood chips in a highly alkaline liquor which selectively dissolves lignin and releases the cellulosic fibers from the wooden matrix. The two major chemicals in the liquor are sodium hydroxide and sodium sulfide. Sodium sulfide, also a strong alkali, readily hydrolyses in water producing sodium hydroxide. The sulfidity is the amount of sodium sulfide in solution divided by the total amount of sodium sulfide and sodium hydroxide. The sulfidity is usually expressed as a percentage which varies between 20 and 30% in pulping liquors. The total amount of sodium hydroxide in solution is called effective alkali (EA) before pulping or residual effective alkali (REA) after pulping. Timely knowledge of the REA ensures good control of the pulping process.
At the beginning of the kraft process white liquor is fed to the digester. This liquor contains a high amount of effective alkali. At the exit of the digester the spent liquor or black liquor is extracted from the digester. This black liquor contains low levels of effective alkali. Black liquor also contains large amounts of organic compounds which are burned in a recovery furnace. The mass of inorganic residues, called smelt, is then dissolved to form green liquor having a low concentration of effective alkali and a high concentration of sodium carbonate. White liquor is then regenerated from the green liquor by causticizing the carbonate through the addition of lime. After the recausticizing operation, a small residual amount of sodium carbonate is carried over to the digester. The total amount of sodium hydroxide, sodium sulfide and sodium carbonate is called the total titratable alkali. The causticizing efficiency (CE) is usually defined as the difference in the amounts of sodium hydroxide between the white and green liquors divided by the amount of sodium carbonate in the green liquor. Sodium sulfate and sodium thiosulfate, together with sodium carbonate, represent a dead load in the liquor recycling system. Sodium thiosulfate is particularly undesirable in processed liquors because of the potential for corrosion of metal surfaces in contact with these liquors. The reduction efficiency (RE) is defined as the amount of green liquor sodium sulfide, divided by the combined amounts of sodium sulfide and sodium sulfate in either green liquor or the smelt. A reduction in dead load chemicals has a beneficial impact on kraft mill operations, thus there is a need for better control of all aspects of kraft mill operations and more efficient use of all chemicals involved in the process. The timely knowledge of the white liquor charge of effective alkali and black liquor charge of residual effective alkali would close the control loop in the digester and minimize alkali and lime consumption.
Various methods of measuring effective alkali have been proposed, however most of these measurements have to be corrected for temperature effects and interferences by other cations and anions, as well as organics. Conductivity sensors have been implemented in some mills, and these give indirect measurement of effective alkali. They may be suitable for on-line measurements of effective alkali in white or green liquors, however they are not suitable for black liquor due to a high solids content and the presence of salts from weak organic acids in the black liquor. On-line measurements of effective alkali in black liquors have been attempted in a number of ways ranging from on-line calorimeters to on-line automatic conductimetric titration methods. None of those systems is straightforward. Titration methods encounter maintenance problems, thus most mill site measurements still rely on laboratory standard methods involving precipitating carbonate and phenoxide ions with barium chloride before performing the titration.
The control of continuous digesters is performed by keeping the chip and white liquor feeds at preset levels which are determined by the overall production rate. Control is performed by adjusting the temperature profile of the cook through the H-factor and determining the resultant blow line kappa number. Kappa number is a measure of pulp lignin content. One disadvantage of this method is that it assumes uniform chip moisture content and digester temperature. Since the pulp must be analyzed in the laboratory for lignin content, there is always a delay in controlling the process.
Other methods to analyze organic content have been developed to correlate the amount of organics with pulp yield and kappa number. On-line methods however have not been entirely satisfactory. In U.S. Pat. No. 4,743,339 Faix et al proposes a method for determining effective alkali in black liquor based upon on-line infrared circular attenuated reflectance measurements. Faix et al also report [TAPPI proceedings, 1989 Wood and Pulping Chemistry Symposium, Raleigh, N.C.] that one is able to measure the consumption of sodium sulfite and the appearance of lignosulfonates during alkaline sulfite anthraquinone methanol pulping, but the results were not very accurate because of spectral non-linearities due to the precipitation of dissolved compounds. Certain limitations were therefore found with this method.
Michell in TAPPI Journal 73(4) 235, 1990, suggested a similar method for kappa number determination by correlating the increase in the integrated band intensity at a wave number of 1118 cm.sup.-1 with decreasing kappa number. Unfortunately, this region is also prone to interferences from the primary and secondary hydroxyl groups in carbohydrates. No attempts were made by either Michell or Faix et al to evaluate the spectral region situated between wave numbers of 1800 to 2900 cm.sup.-1 for useful information.
To be useful the direct measurement of effective alkali in process liquors must be free of interferences from both inorganic and organic compounds. Up to now, no infrared spectrophotometric method for directly measuring effective alkali in pulping liquors has been developed and implemented for routine use in pulp and paper manufacturing.